The present invention relates to a nickel positive electrode for use in alkaline storage battery such as nickel-metal hydride storage battery, nickel-cadmium storage battery and so on, and a method for producing the same.
With the current rapid and wide spread of information equipment such as portable phone, PHS, notebook-type personal computer, etc., there is a serious demand for a secondary battery that has a high energy density and exhibits excellent performance as a battery even at high ambient temperatures due to heat generation by such equipment. There has been another demand for the development of a novel secondary battery with a high energy density as a power source for electric vehicle which can tolerate its use in a wide range of ambient temperatures. In order to answer such demand, a provision of a high capacity to the nickel-cadmium storage battery using a conventional sintered nickel positive electrode has been realized in the field of nickel-cadmium storage battery, and a nickel-cadmium storage battery having a high energy density including a foamed metal nickel positive electrode which has a 30 to 60% higher capacity than the former electrode has been developed. Furthermore, a nickel-metal hydride battery having a higher capacity than the nickel-cadmium storage battery which includes a hydrogen storage alloy as the negative electrode has also been developed.
The above-noted various high capacity alkaline storage batteries include a sintered porous nickel substrate, a three-dimensional foamed porous nickel substrate of high porosity (90% or more) or a porous nickel fiber substrate being filled with a nickel hydroxide powder at a high density. The use of such porous substrates has led to improvements of the energy density: Compared to 400 to 500 mAh/cm.sup.3 of the conventional sintered nickel positive electrode, the recent sintered nickel positive electrode affords 450 to 500 mAh/cm.sup.3 and the foamed metal nickel positive electrode affords 550 to 650 mAh/cm.sup.3. In correspondence with realization of high capacities, the positive electrode has been improved in various characteristics such as utilization and performance at high temperatures with the wide use of a variety of additives. Under the circumstances, there have been previous proposals as follows:
(1) Incorporate a compound of an element selected from among yttrium, indium, antimony, barium and beryllium in the positive electrode (see Japanese Laid-Open Patent Publication No. Hei 6-103973). PA1 (2) A method to coat a nickel hydroxide powder as active material with .beta.-Co(OH).sub.2 (see Japanese Laid-Open Patent Publication No. Sho 61-110962); PA1 (3) A method to adhere CoO or .beta.-Co(OH).sub.2 particles to a nickel hydroxide powder active material by electrical charge and subsequently fix it thereon by mechanical impact (see Japanese Laid-Open Patent Publication No. Hei 1-281670); and PA1 (4) A method to form a layer of a cobalt compound having a valence larger than 2 which has a disordered crystal structure on the surface of nickel hydroxide (see Japanese Laid-Open Patent Publication No. Hei 8-148146).
Various additives other than the above have already been used in order to improve the battery performance. An addition of cobalt or a cobalt compound is the most widely applied method in order to improve the utilization of nickel hydroxide and there are many proposed methods for adding cobalt or a cobalt compound as follows:
The intended effects of the method (1) are to increase the oxygen evolution overvoltage as a competitive reaction against charge reaction at high ambient temperature by adsorption of a compound of yttrium, indium, antimony or the like onto the surface of the active material nickel hydroxide thereby improving charge efficiency and active material utilization at high ambient temperature.
This method, however, has a drawback that simple application of this method only does not offer the expected effects due to non-homogeneous distribution of the additive in the positive electrode. In order to have a prominent effect, the additive must be included in large amounts, which hinders realization of a high capacity battery.
The methods (2), (3) and (4) propose how to add cobalt in order to improve the utilization of nickel hydroxide. Those methods, however, are disadvantageous in that they require much labor in making Co(OH).sub.2 or CoO coated on or adsorbed onto the nickel hydroxide powder and have a very high production cost, which hinders industrial application of these methods.
Moreover, addition of cobalt or a cobalt compound has been applied separately apart from the addition of ZnO for improving storage characteristics of the resultant battery or the addition of the compound proposed by the method (1) and therefore there remains much room for improvement in terms of effective combinations of plural additives.